Process and apparatus for the production of single crystal compounds



March 15, 1966 w. R. DERBY ETAL 3,240,568

PROCESS AND APPARATUS FOR THE PRODUCTION OF SINGLE CRYSTAL COMPOUNDS Filed Dec. 20, 1961 INVENTORS WALLENE RDERBV ARNO H. HERZOG United States Patent F 3,24%,568 PROCEES AND APPARATUS FUR THE PRUDUC- TEIUN @F SINGLE CRYSTAL COMPOUNDS Wallene R. Derby, Glendale, and Arno H. Herzog, St.

Louis, Mo, assignors to Monsanto Company, a corporation of Delaware Filed Dec. 20, 19611, Ser. No. 160,712 14 Claims. (Cl. 23-301) This invention relates to a method and apparatus for the production of large single crystals of inorganic compounds.

It is an object of this invention to provide a new, simple and economical method and apparatus for the production of large single crystals of inorganic compounds by a re-crystallization procedure.

It is a further object of this invention to provide a novel crystallizing container for obtaining large single crystal compounds by the above procedure.

Further objects and advantages of the present invention will become apparent as the description of the invention proceeds.

In FIGURES 1, la, 1b and 2 are shown various views of the apparatus employed in the present invention.

In FIGURES l and la are shown, respectively, top and side views of the preferred crystallizing container of this invention.

In FIGURE lb is shown a cross sectional view of the crystallizing container shown in FIGURE 1 from a cut at line A.

FIGURE 2 shows a sectional view of the crystallizing container in place in a typical crystallizer furnace arrangement.

Of particular interest in the present invention is the production of large single crystal compounds formed from elements of Groups IliB and VB of the periodic system (Hubbards Chart of the Atoms). These compounds are designated herein as III-V compounds. Compounds within this group which are contemplated herein include arsenides, phosphides and atimonides of aluminum, gallium and indium. Other compounds contemplated by this invention include the II-Vl and i-VII compounds, e.g., the sulfides, selenides and tellurides of zinc, cadmium and mercury and the chlorides, bromides and iodides of sodium, potassium, copper, rubidium, silver, cesium and gold.

In recent years great demand has arisen in the semiconductor field for single crystal III-V compounds which have been found to be excellent substitutes for the elements of Group IVB (carbon, silicon, germanium and tin) which have gained prominence as semiconductors in numerous electronic devices. Carbon and tin which are semiconductors only in the diamond-lattice modification, are of more scientific interest than practical utility because of the difiiculty of producing synthetic diamond and the instability of tin in its diamond-lattice modification. For practical applications silicon and germanium have been used extensively. However, the high cost of germanium and the difficulty of purifying silicon have made the use of TIL-V compounds particularly desirable as substitutes for silicon and germanium in semiconductor applications.

Various prior art methods are known for the production of polycrystalline forms of III-V compounds. However, for industrial applications, the monocrystalline structure of these compounds is most desired, and several methods have been devised for obtaining single crystal III-V compounds. Among such methods are the melting and normal freezing of the compound, zone melting, gradient freezing and crystal pulling procedures.

Of the foregoing procedures the gradient freeze method 3,240,568 Patented Mar. 15, 1966 is the most desirable from the standpoint of simplicity, economy and highest yield of product. However, crystallization by this method to be efiicient must be performed in a meticulously controlled temperature environment having uniform temperature gradients within the crystallizer furnace, within the container holding the molten compound and within the melt itself. In short, there must be a completely symmetrical temperature environment in the crystallization system in order to prevent or minimize the opportunity for polycrystallization of the melt.

Crystal formation and growth is extremely sensitive to non-uniform temperature gradients in the system, hence, in the gradient freeze method, for example, disturbances in the temperature gradient across the melt or the crystallizing container give rise to the formation of crystal nuclei at the site of the temperature disturbance. Usually, these sites of non-uniform temperature gradients arise between the container holding the melt and the melt itself. Disturbances within the temperature environment surrounding the container may be caused by uneven heating of the crystallizing zone, by cooling the melt too fast or for other reasons.

It is the purpose of the present invention to provide an improved method and container for the gradient freeze crystallization of inorganic compounds, particularly III-V compounds. It has now been found that single crystal IHV compounds having good semiconductor electrical properties can be grown in high yield by the use of a novel dual-container crystallizer.

Typical dual container crystallizers contemplated herein include elongated double boat containers as illustrated in FIGURES 1, la and lb of the drawing and comprise a smaller inside boat 5 and a larger outside boat 6 having generally, but not necessarily, the same geometrical shape as the inner boat. Broadly, the dual container is of greater length than depth and breadth. The significance of this geometry will be discussed hereinafter.

The inner boat has a downwardly concave bottom with sides and ends rising upwardly and outwardly from, and as uninterrupted, continuous extensions of the bottom, thus forming the inner walls of the boat. The upward curvatures of the sides and ends of the internal surfaces of the boats are designed to accommodate and relieve the strain created by crystal expansion during crystallization; boats having vertical walls tend to crack and/or break under this strain. The inner surfaces of the boats are generally smooth, since surface irregularities and angles form sites for nucleation. However, in cases where the contact angle between the melt and the boat is very large, sandblasted boats can suitably be used. Suitable refractory materials for these boats include quartz or fused silica.

The inner boat is suspended inside the outer boat, conveniently in such manner that an annular space 9 exists between the inner and outer boats. Tie bar 7, suitably made of quartz, is welded to the upper edges near one end of both boats. A rigid attachment such as this is necessary in order to maintain the position of the inner boat as shown because (1) the density of the molten compound in the annulus is generally, greater than the quartz inner boat which tends to be buoyed up by the compound, and (2) the crystallizing compound in the annulus exerts considerable crystal strain which also tends to displace the inner boat. However, due to the rigid attachment of tie bars 7, at one end and additional support on the sides of the inner boat by the compound crystallizing in the annulus, rigid attachment of the two boats is not necessary at the opposite end, and cross-bar 8, suitably made of quartz, which is welded to the inner boat, merely rests upon the upper surface edges of the outer boat.

In a preferred embodiment the inner boat has a small restricted opening 10 in the lower portion of one end leading into the annular space between the boats. The purpose of the restricted opening, a small hole and/ or slit, will be explained hereinafter,

In operation, a polycrystalline form of the compound to be converted into the monocrystalline form is charged into the inside boat, in the preferred embodiment. The container is placed in a substantially horizontal position into a quartz (or other refractory material) reactor tube which is then placed into a crystallizer furnace and the compound is heated to above its melting point. As the compound melts it flows from the inner boat through the restricted opening into the annular space surrounding the inner boat until the entire charge is melted and the surface level of the molten compound is substantially the same inside the inner boat and in the annular space. A horizontal temperature gradient is maintained across the melt by means of auxiliary heaters in or around the hot end of the reactor or by means of electrical windings of increasing resistance in the crystallizer furnace, or by placing the boat in the furnace in such position that one end of the boat is near the center of the furnace, or at a higher temperature, while the other end of the boat is closer to one end of the furnace, or at a lower temperature level.

The crystallizer furnace is then allowed to cool from one end at a slow and uniform rate to begin the crystallization cycle. The molten compound in the annular space, being closer to the cooling front in the furnace than the melt in the inner boat, begins to crystallize first. A crystallization front moves "along the melt in the annular space toward and around the inner boat. As the crystallization front proceeds around the inner boat and as cooling continues the melt inside the inner boat also begins to crystallize. Preferably, the crystallization front proceeds at an angle, the top preceding the bottom. In this manner, if other crystal seeds are formed at the bottom of the melt, they will be trapped and not allowed to grow because of the crystal front projecting over them. Such an angle-front pattern is obtained by maintaining a vertical temperature gradient, preferably 5 to degrees Centigrade lower at the top of the melt. The melt in the annulus between the boats provides an isothermal jacket around the inner boat and prevents polynucleation of melt in the inner boat as crystallization continues. By this means a simple and economic method is provided for growing single crystals in a symmetrical temperature environment.

In a preferred embodiment of the invention, using an inner boat having a restircted opening into the annulus between the boats. Crystallization of the melt in the tioned in the furnace with the restricted opening in the inner boat facing the cold end of the furnace. When the apparatus is positioned with the restricted opening at the cold end of the furnace, the crystallization front in the melt in the annulus between the boats reaches the cold end of the inner boat and a single crystal face enters the restricted opening and continues to grow as a single crystal in the inner boat melt.

In another embodiment of the present invention, the double boat apparatus is positioned in the furnace in such manner that the restricted opening in the inner boat faces the hot end of the furnace, but in this embodiment, the restricted opening functions only to permit the molten compound in the inner boat to flow into the annulus between the boats. Crystallization of the melt in the inner boat is initiated at the inner surface of the cold end of the inner boat by the formation of a single nucleus, which continues to grow as a single crystal as cooling proceeds.

In the preferred embodiment, a small hole, e.g., 7

in diameter with a slit wide merging into the hole, is used. In general the dimensions of the restricted opening must be of such size that the surface tension of the molten compound will not prevent its passage through the opening.

In still another embodiment of the invention, the polycrystalline compound is charged into both the inner boat (which, in this embodiment, has no restricted opening) and the annulus between the boats. Since the width of the annulus may be quite narrow, e.g., of the order of a few millimeters, the compound is charged into the annulus in a powdered form. The amount of powdered compound charged into the annulus is sufiicient, when molten, to have a liquid level at least as high as that of the melt in the inner boat. The double boat apparatus is then placed into a quartz reactor tube which is placed into the crystallizer furnace and the same crystallization procedure described above is followed. Again, the melt in the annulus forms an isothermal jacket surrounding the inner boat preventing or greatly minimizing temperature disturbances and polynucleation of the melt in the inner boat and permitting single crystal growth.

An essential feature of this invention is the provision of an isothermal environment around the inner boat. Typically, this isothermal environment is provided by a liquid jacket of the compound being crystallized. However, it is also contemplated that liquids other than the compound to be crystallized be utilized for this purpose, e.g. a molten metal such as lead can be used. The annulus containing the isothermal material should be of substantially uniform dimensions, particularly throughout the length of the inner boat. To assure a substantially uniform width of the annulus between the inner and outer boats and to keep the inner boat from shifting or floating against the outer boat, tie bars 7, suitably made of quartz, are welded between the inner and outer boats near one end and at the upper level of the boats. These tie bars are, at high temperatures, semi-fluid but are sufficiently rigid to withstand crystal strain and to keep the inner boat aligned with the outer boat, hence, it is not necessary to have tie bars at the opposite end. However, means must be provided for keeping this opposite end of the inner boat from touching the outer boat. Suitably, this is accomplished by welding 21 cross-bar of quartz to the inner boat and resting the ends of the cross-bar on the upper edges of the outer boat.

In a preferred modification of this invention the crystallization procedure is carried out in an atmosphere free of reactive gases, e.g., oxygen. In this modification the double boat apparatus is charged with the compound to be crystallized. The double boat is then placed in a quartz reactor tube closed at one end and the whole assembly placed in a crystallizer furnace. The quartz tube is connected to a vacuum system and evacuated to high vacuum, e.g., of the order of 17 l0* mm. Hg, and the compound degassed at elevated temperatures for several hours, e.g., 500 C. for 812 hours and then 800 C. for 1-5 hours for GaAs. The quartz tube is then sealed off from the vacuum system and the furnace heated to melt the compound which is then crystallized by the gradient freeze method.

A more detailed description of the invention is set forth in the following examples having reference to a preferred embodiment and the accompanying drawings:

Example 1 This example illustrates the production of single crystal gallium arsenide. A double-boat crystallizing container having dimensions of four inches long, 25 mm. wide and 20 mm. deep for inner boat 5, and dimensions of four and one-half inches long, 33 mm. wide and 25 mm. deep for outer boat 6, was charged with 127.4 grams of gallium arsenide, GaAs. The charged container was then placed in a quartz reactor tube 15 shown in FIGURE 2. Tube 15 has an extension on one end terminating in bulb 17 which serves as vapor control reservoir. Tube is then placed in a crystallizer furnace 18 butted end to end with vapor temperature control furnace 19. Alternatively, these two furnaces may be substituted by a single furnace having two or more sections with separate temperature control means for each section. Crystallizer furnace 18 is sealed with firebrick 20 and the vapor control furnace 19 sealed with firebrick 21. Tube -15 was then connected to a vacuum source (not shown) connected at point 16, and the GaAs degassed at a pressure of 7x10 mm. Hg for about 12 hours at 500 C., then for 4 hours at 800 C. After the degassing operation, the tube was sealed off from the vacuum source at point 16. Crystallizer furnace '18 was then heated until control thermocouple 24 registered 1295" C. and thermocouples 22 and 23, placed against the reactor tube at points near the ends of the double boat container, registered respectively, 1244 C. and 1285 C. At these temperatures the gallium arsenide charge was completely molten. Meanwhile, vapor control furnace 19 was heated to a temperature sufliciently high to achieve an arsenic pressure of about 0.9 to 1.0 atmosphere. Thermocouples 27, 2 8 and 29, recording the arsenic temperature, read 626 C., 616 C., and 633 C., respectively.

In heating the gallium arsenide, arsenic vapor is released and fills the reactor tube. This arsenic vapor exerts a partial pressure which, during the subsequent crystallization cycle, is maintained substantially at the dissociation pressure of gallium arsenide at its melting point, i.e., about 0.9 to 1.0 atmosphere, in order to obtain a product having sto-ichiometric proportions. In the case of other III-V compounds, the vapor pressure of the more volatile component of the compound, i.e., the Group V element is similarly maintained at the dissociation pressure of the compound at its melting point, although the specific pressure will be different for each compound.

In the case of other compounds which are stable at their melting points, eg a I-VIII compound such as cupric chloride, it is not a critical feature, as in the case of IIIV and II-VI compounds, to maintain the vapor pressure of the system at the dissociation pressure of the compound at its melting point.

The arsenic (or other volatile vapor) temperature is controlled by furnace 19 and recorded by thermocouples 27, 28 and 29. Vapor pressures in excess of that recited above result in a ballooning of the reactor tube 15, or if the vapor pressure is substantially lower, the reactor tube collapses. Also, non-stoichiometric products rich in the vapor element may result if the vapor pressure is too high, or inclusions of the free Group III metal result when the vapor pressure is too low.

Since the vaporized element, arsenic in this example, must react with all the molten metal, here, gallium, in order to obtain stoichiometric gallium arsenide, it is essential that the arsenic reach the gallium at the lower levels of the melt. To facilitate accomplishment of this essential, the dual container is designed to hold a melt having a substantially low head or liquid level. Suitably, boats which are relatively long, shallow and narrow are preferred, although numerous variations in the geometry of the container are contemplated.

The gallium arsenide upon melting flowed from inner boat 5 through the restricted opening 10, a hole 3 mm. in diameter, into annulus 9, until the liquid level in the annulus was substantially the same as that in the inner boat.

When the control thermocouple 24 registered the above temperature, an automatic turn-down mechanism was adjusted to permit the crystallizer furnace to cool at a uniform rate of 3.7 C./hr. Cooling rates up to 10 C./hr. may be used, but a too-fast cooling rate can result in polynucleation in the melt and/or breaking of the boats. While the crystallizer furnace was cooling, the vapor control furnace 19 was held at substantially the same temperatures recited above.

Crystallization of the gallium arsenide melt began in that portion of the melt in the annulus surrounding the inner boat at the colder end. Since the upper portion of the melt is maintained at from 5l0 C. colder than the bottom of the melt by external resistance in the upper windings of the crystallizer furnace, and recorded by thermocouple 25, the crystallization front moves along the melt at an angle, the top preceding the bottom. As cooling continued the crystallization front moved to and around the inner boat following the melt in the annulus. A single crystal face of gallium arsenide entered into the inner boat 5 through the tiny hole 10 in the lower portion of the boat and continued to grow as a single crystal cooling proceeded. Single crystal formation and growth also occurs without the restricted opening, initiating at the inner surface of the inner boat.

Crystallization continued throughout the melt, both in the annulus between the two boats and in the inner boat, until the entire melt had solidified. At this time the automatic turn-down mechanism for the crystallizer furnace adjusted to a faster cooling rate of 200 C./hr. This rate may be as high as 300 C./hr., but a toofast cooling rate may result in a cracking of boats. When the temperature in the crystallizer furnace had reached the natural cooling rate of the furnace, about 620 C., both furnaces were turned off and cooled to atmosphere.

The ingot was removed from the inner boat and found to be entirely monocrystalline in structure, although the gallium arsenide in the annulus was highly polycrystalline. The single crystal ingot weighed 37.6 grams representing a yield of 29.6% based upon the 127.4 grams starting material. Yields up to 66% have been obtained by this process. These percentages represent very high yields of single crystals compared to those of prior art gradient freeze methods which range from 5 to 15% or less.

The single crystal gallium arsenide obtained in this example had the following electrical properties:

n represents net carriers per cc. R represents Hall eoeflicient em /coulomb. p represents reslstlvlty ohm/cm.

and

R/p represents mobility (cmfi/volt sec.)

Example 2 The procedure described in Example 1 was repeated, except that the double boat crystallizer was turned around inside the reactor tube 5, i.e., the restricted opening 10 facing the hot end (right side in FIGURE 2) of the furnace 18.

The heating and cooling procedures of Example 1 was repeated, except this time single crystal nucleation initiated on the inner surface of the inner boat instead of by a single crystal of GaAs appearing through the restricted opening which, in this example, serves only to permit the molten GaAs to flow from the inner boat into the annulus surrounding the inner boat, thus providing thermal insulation for the melt in the inner boat.

The ingot obtained by the procedure in this example was single crystal GaAs and had electrical properties similar to those described in Example 1.

Example 3 This example illustrates the preparation of singly crystal indium phosphite according to the method described in Example 1.

The same procedure described in Example 1 is repeated, except that the inner boat 5 is charged with grams of indium phosphide, InP. After the sample is degassed, the crystallizer furnace 18 is heated until control thermocouple 24 records 1095 C. and thermocouples 22 and 23 record 1070 C. and 1090 C., respectively. The In? charge is completely molten at these temperatures. While the crystallizer furnace is being heated to these temperatures, the phosphorus vapor control furnace is being heated to a temperature sufficiently high to achieve a phosphorus pressure of about 20 atmospheres. Thermocouples 27, 28 and 29 register 546 C., 542 C. and 545 C., respectively. The phosphorus pressure is maintained substantially constant during the subsequent crystallizing cycle.

When control thermocouple 24 records 1095 C., the automatic turn-down mechanism controlling the crystallizer furnace adjusts to permit the furnace to cool at a uniform rate of 4 C./hr. Crystallization of the InP melt proceeds as described in Example 1, this time a single crystal of InP entering from the annulus between the boats into the inner boat through a small slitted-opening and continuing to grow as a single crystal therein. When the entire InP melt has crystallized, the cooling rate is then increased to about 200 C./hr. When the crystallizer furnace temperature has reached the natural cooling rate of the furnace about 620 C., both the crystallizer and vapor control furnaces are turned ofi? and cooled to atmosphere.

When commercially available indium and phosphorus are purified, combined and remelted as described in this example, single crystal InP is produced with a maximum carrier concentration of /cm. and room temperature mobilities of 3500-4000.

Example 4 In this example, that modification of the invention as described in Example 2 is repeated using, however, in-. dium phosphide instead of gallium arsenide and the temperature and pressure conditions described in Example 3.

The indium phosphide ingot obtained in this manner is likewise single crystalline in form and has electrical properties similar to those recited in Example 3.

Example 5 This example illustrates the preparation of single crystal indium antimonide according to the method as described in Examples 1 and 3.

The same procedure described in Examples 1 and 3 is repeated, except that the inner boat 5 is charged with 150 grams of indium antimonide, InSb. After the sample is degassed, the crystallizer furnace is heated until control thermocouple 24 records 550 C. and thermocouples 22 and 23 record 525 C. and 545 C., respectively. The InSb is completely molten at these temperatures. In this example, the tube extension leading into the low temperature furnace is sealed off at a point inside furnace 18, since the vapor pressure over InSb is very low and there is no need to control the Sb pressure.

When control thermocouple 24 registers 550 C., the automatic turn-down mechanism controlling the crystallizer furnace adjusts to permit the furnace to cool at a uniform rate of 4 C./hr. Crystallization of the InSb melt proceeds as described in Example 1, this time a single crystal of InSb entering from the annulus between the boats into the inner boat through a small slittedopening and continuing to grow as a single crystal therein. When the entire InSb melt has crystallized, the cooling rate is then increased to about 200 C./hr. When the crystallizer furnace temperature has reached the natural cooling rate of the furnace, about 620 C., the crystallizer furnace is turned off and cooled to atmosphere.

The InSb ingot obtained according to this example is found to be single crystalline in form having good electrical properties, suitable for use in semiconductor applications.

Example 6 In this example, that modification of the invention as described in Examples 2 and 4 is repeated using, however, indium antimonide and the temperature and pressure conditions described in Example 5.

The indium phosphide ingot obtained in this manner is likewise single crystalline in form and also has electrical properties suitable for semiconductor applications.

Example 7 This example illustrates the preparation of single crystal indium arsenide according to the method as described in Examples 1, 3 and 5.

The same procedure described in Example 1 is repeated, except that the inner boat 5 is charged with grams of indium arsenide, InAs. After the sample is degassed, the crystallizer furnace is heated until control thermocouple 24 records 980 C. and thermocouples 22 and 23 record 945 C. and 970 C., respectively. The InAs charge is completely molten at these temperatures. While the crystallizer furnace is being heated to these temperatures, arsenic vapor control furnace is being heated to a temperature sufiiciently high to achieve a phosphorus pressure of about 0.5 atmosphere. Thermocouples 27, 28 and 29 register 553 C., 551 C. and 550 C., respectively. The arsenic pressure is maintained substantially constant during the subsequent crystallizing cycle.

When control thermocouple 24 registers 980 C., the automatic turn-down mechanism controlling the crystallizer furnace adjusts to permit the furnace to cool at a uniform rate of 4 C./hr. Crystallization of the InAs melt proceeds as described in Example 1, this time a single crystal of InAs entering from the annulus between the boats into the inner boat through a small slitted-opening and continuing to grow as a single crystal therein. When the entire InAs melt has crystallized, the cooling rate is then increased to about 200 C./ hr. When the crystallizer furnace temperature has reached the natural cooling rate of the furnace, about 620 C., both the crystallizer and vapor control furnaces are turned off and cooled to atmosphere.

The InAs ingot obtained according to this example is found to be single crystalline in form having the following average electrical properties:

n=2.6 X 10 R: -287 Mobility=24,000 (cm. /volt sec. at 300 C.)

Example 8 In this example, that modification of the invention as described in Examples 2, 4 and 6 is repeated using, however, indium arsenide and the temperature and pressure conditions described in Example 7.

The indium arsenide ingot obtained in this manner is likewise single crystalline in form and has electrical properties similar to those recited in Example 7.

Single crystal compounds having 11 or p-type conductivity are prepared intrinsically, or by incorporation into the compound, prior to remelting, of the desired amount and kind of dopant. To prepare p-type III-V compounds, for example, a Group II element such as beryllium, magnesium, zinc, cadmium or mercury is added prior to recrystallization. N-type conductivity is obtained by adding a Group VI element such as sulfur, selenium or tellurium. These dopants may be added by any means known to the art, e.g. by diffusing the dopant into the compound, by alloying, or by adding the dopant to the component(s) used in preparing the compound.

In preparing doped materials using the apparatus described herein, care must be used to maintain the temperature of the vapor control reservoir above the dew point of the dopant or any compound thereof formed with the more volatile component of the recrystallizing compound. Alternatively, the vapor control reservoir (the extension and bulb 17 of tube 15 in FIGURE 3) is sealed off and the only conditions to observe are that the temperature and pressure in the reactor tube 15 not be substantially higher than the decomposition pressure of the compound at its melting point, or high enough to explode the reactor tube.

Example 9 This example illustrates the preparation of n-type indium arsenide according to this invention.

The procedure described in Example 7 is repeated, except that to the undoped indium arsenide charge is added a calculated quantity of an indium arsenide ingot doped with sulfur to a concentration of 1.03 X10 carriers/cnfi.

The indium arsenide ingot obtained according to this procedure, under the temperature and pressure conditions recited in Example 7 has the following average electrical properties:

22:9.1 X 10 carriers/cm. R=0.8 Mobility=5,500 (cmF/volt sec. (300 C.))

A modification of the foregoing examples is to form the compound directly in the inner boat. For example, gallium metal is loaded into the inner boat, melted and then arsenic is vaporized at a remote section of the reactor tube, e.g., from reservoir 17 in FIGURE 2. The gallium and arsenic then react to form gallium arsenide which is then crystallized as described above.

Single crystals of III-V compounds e.g., the phosphides, arsenides and .antimonides of aluminum, gallium and indium, and II-V I and I-VII compounds as mentioned earlier are produced in the same manner disclosed in the preceding examples, with only the temperatures and vapor pressures varying. The process is of Wide application and may be performed with any compounds crystallized by the gradient freeze method.

Various modifications of this invention will occur to those skilled in the art Without departing from the spirit and scope thereof.

What is claimed is:

ll. As an article of manufacture an elongated, horizontal gradient freeze crystallizing container comprising:

(a) an inner section of greater length than width and depth and having sides and ends which rise upwardly and outwardly as continuous extensions of a downwardly concave bottom, said sides and ends having upper edges which define an open top for said section (b) an outer section having substantially the same geometrical configuration as said inner section (c) an annular space between (a) and (b) and surrounding (a) on all sides and bottom ((1) means of suspension attachment to and support by (b) of (a), said means comprising tie-bar means fixedly secured to the sides of both said sections adjacent the upper edges of one end thereof and a crossbar means fixedly secured to the inner section adjacent its upper edges and resting on the upper edges of the outer section, and

(e) a restricted opening in (a) leading into (c), said opening being of such size that the surface tension of a molten compound selected from the group consisting of I-VII, II-VI and HLV compounds will not prevent its passage therethrough at substantially the temperature of crystallization.

2. Article according to claim 1 wherein said restricted opening comprises a small slit merging into a small hole.

3. Apparatus for producing single crystal compounds which comprises in combination:

(1) an elongated gradient freeze crystallizing container comprising:

(a) an inner section,

(b) an outer section,

() an annular space between (a) and (b) and surrounding 1) on all sides and bottom, and

(d) means of suspension attachment to and support y of (e) a restricted opening in (a) leading into (c),

said inner section being of greater length than width and depth and having sides and ends which rise upwardly as continuous extensions of a downwardly concave bottom, said outer section having substantially the same geometrical configuration as said inner section and said opening being of such size that the surface tension of a molten compound selected from the group consisting of IVII, IIVI and III-V compounds will not prevent its passage therethrough,

(2) an elongated refractory crystallizing zone in which (1) is horizontally disposed,

(3) means for obtaining a vacuum in (2),

(4) means for producing temperature gradients vertically and longitudinally across (1),

(5) means for controlling vapor pressures in (2), and

(6) means for controlling cooling rates in (2).

4. Apparatus according to claim 3 wherein said restricted opening faces the hotter end of said longitudinal temperature gradient.

5. Apparatus according to claim 3 wherein said restricted opening faces the colder end of said longitudinal temperature gradient.

6. Process for the production of single crystal compounds selected from the group consisting of IVII, II- VI and III-V compounds which comprises:

( 1) providing a melt of a polycrystalline form of one of said compounds in an elongated, horizontal crystallizing zone comprising (a) an inner section, (b) an outer section surrounding (a) on all sides and bottom and having substantially the same geometrical configuration as (a), and (c) an annular melt-containing space between (a) and (b) which thermally insulates (a), said inner section having a restricted opening into (0) and being of greater length than Width and depth and having sides and ends which rise upwardly and outwardly as continuous extensions of a downwardly concave bottom, said opening being of such size that the surface tension of said compound will not prevent its passage therethrough,

(2) adjusting the temperatures within said crystallizing zone to provide vertical and horizontal temperature gradients,

(3) cooling said crystallizing zone incrementally from one end at a slow and uniform rate to initiate crystallization of said melt in (c) and progressively move a crystallization front therein toward and around (a),

(4) initiating single crystal nucleation in thermally insulated (a) by continuing cooling along said crystallizing zone, and

(5) continuing said cooling until the entire melt in said crystallizing zone has crystallized, and thereafter recovering a single crystal form of said compound from said inner section.

7. Process for the production of a single crystal form of compounds selected from the group consisting of I-VII, II-VI and Ill-V compounds which comprises:

(1) heating a polycrystalline form of one of said compounds to form a melt thereof in an elongated, horizontal gradient freeze crystallizing zone comprising: (a) an inner section, (b) an outer section surrounding (a) on all sides and bottom and having substantially the same geometrical configuration as (a), and (c) an annular melt-containing space between (a) and (b) which thermally insulates (a), said inner section having a restricted opening into (c) and being of greater length than width and depth and having sides and ends which rise upwardly and outwardly as continuous extensions of a downwardly concave bottom, said opening being of such size that the surface tension of said compound will not prevent its passage therethrough,

(2) adjusting the temperatures within said crystallizing zone to provide vertical and longitudinal temperature gradients within said zone,

(3) cooling said crystallizing zone from one end incre mentally at a slow and uniform rate to initiate crystallization of said melt in (c) and progressively move a crystallization front therein toward and around (a),

(4) initiating single crystal nucleation in thermally insulated (a) by continuing cooling along said crystallizing zone, and

() continuing cooling until the entire melt in said crystallizine zone is crystallized, and thereafter recovering a single crystal form of said compound from (a).

8. Process according to claim 7 wherein said polycrystalline compound is charged into said inner section of said crystallizing zone and which, upon heating and melting flows into said annular space through said restricted opening until the melt level in said inner section and annular space is substantially equal.

9. Process according to claim 8 wherein said single crystal nucleation is initiated by the entry of a single crystal face of said compound through said restricted opening into said inner section.

10. Process for the production of a single crystal form of compounds selected from the group consisting of I- VII, II-VI and III-V compounds which comprises:

(1) heating a polycrystalline form of one of said compounds to form a melt thereof in an elongated, horizontal gradient freeze crystallizing zone comprising (a) an inner section, (b) an outer section surrounding (a) on all sides and bottom and having substantially the same geometrical configuration as (a), and (c) an annular melt-containing space which thermally insulates (a), said inner section having a restricted opening into (c) and being of greater length than width and depth and having sides and ends which rise upwardly and outwardly as continuous extensions of a downwardly concave bottom, said opening being of such size that the surface tension of said compound will not prevent its passage therethrough,

(2) adjusting the temperatures within said crystallizing zone to provide vertical and longitudinal temperature gradients within said zone,

(3) adjusting the vapor pressure over said melt to substantially the decomposition pressure of said polycrystalline compound at its melting point,

(4) cooling said crystallizing zone incrementally from one end at a slow and uniform rate to initiate crystallization of said melt in (c) and progressively move a 12 crystallization front therein toward and around (a), (5 initiating single crystal nucleation in thermally insulated (a) by continuing cooling along said crystallizing zone, and (6) continuing cooling until the entire melt in said crystallizing zone is crystallized and thereafter recovering a single crystal form of said compound from (a).

11. Process according to claim 10 wherein said polycrystalline compound is charged into said inner section of said crystallizing zone and which, upon heating and melting, flows into said annular space through said restricted opening until the melt level in said inner section and said annular space is equal.

12. Process according to claim 11 wherein said single crystal nucleation is initiated by the entry of a single crystal face of said compound through said restricted opening into said inner section.

13. Process according to claim 12 wherein said polycrystalline compound is a III-V compound.

14. Process according to claim 13 wherein said III-V compound is gallium arsenide.

References Cited by the Examiner UNITED STATES PATENTS 2,686,212 8/1954 Horn 23-301 2,876,083 3/1959 Prietl 23-295 2,892,739 6/1959 Rusler.

3,041,133 6/1962 Hicks et al. 23-273 3,078,151 2/1963 Kappelmeyer 23-273 OTHER REFERENCES Lawson et al.: Preparation of Single Crystals, QD 931 L3, 1958a, 02, 1958, pages 16 to 18.

Pfann: Zone Melting, pages 62 to 66, 1958, TP 156 Z6P4 C. 2.

Growth of Gallium Arsenide by Horizontal Zone Melting, by Richards, Journal of Applied Physics, vol. 31, No. 3, March 1960, pages 600603.

Semiconductors, by Hannay, pages to 99, Reinhold Publishing Corporation, New York.

ROBERT F. BURNETT, Primary Examiner.

NORMAN YUDKOFF, ANTHONY SCIAMANNA,

Examiners. 

1. AS AN ARTICLE OF MANUFACTURE AN ELONGATED, HORIZONTAL GRADIENT FREEZE CRYSTALLIZING CONTAINER COMPRISING: (A) AN INNER SECTION OF GREATER LENGTH THAN WIDTH AND DEPTH AND HAVING SIDES AND ENDS WHICH RISE UPWARDLY AND OUTWARDLY AS CONTINUOUS EXTENSIONS OF A DOWNWARDLY CONCAVE BOTTOM, SAID SIDES AND ENDS HAVING UPPER EDGES WHICH DEFINE AN OPEN TOP FOR SAID SECTION (B) AN OUTER SECTION HAVING SUBSTANTIALLY THE SAME GEOMETRICAL CONFIGURATION AS SAID INNER SECTION (C) AN ANNULAR SPACE BETWEEN (A) AND (B) AND SURROUNDING (A) ON ALL SIDES AND BOTTOM (D) MEANS OF SUSPENSION ATTACHMENT TO AND SUPPORT BY (B) OF (A), SAID MEANS COMPRISING TIE-BAR MEANS FIXEDLY SECURED TO THE SIDES OF BOTH SAID SECTIONS ADJACENT THE UPPER EDGES OF ONE END THEREOF AND A CROSSBAR MEANS FIXEDLY SECURED TO THE INNER SECTION ADJACENT ITS UPPER EDGES AND RESTING ON THE UPPER EDGES OF THE OUTER SECTION, AND (E) A RESTRICTED OPENING IN (A) LEADING INTO (C), SAID OPENING BEING OF SUCH SIZE THAT THE SURFACE TENSION OF A MOLTEN COMPOUND SELECTED FROM THE GROUP CONSISTING OF I-VII, II-VI AND III-V COMPOUNDS WILL NOT PREVENT ITS PASSAGE THERETHROUGH AT SUBSTANTIALLY THE TEMPERATURE OF CRYSTALLIZATION. 